1. Field of the Invention
This invention relates to a method for producing hydrogen peroxide, and more specifically, this invention relates to a method for producing hydrogen peroxide via the utilization of advanced membrane technology
2. Background of the Invention
Environmental considerations and regulations continue to prompt industries to use compounds which are less detrimental to the ecosystem. One example is the dramatic increase in the use of hydrogen peroxide for pulp and paper industry applications. Instead of relying on chlorine and chlorine dioxide bleaching processes, many paper producers now utilize chemithermo-mechanical pulping where hydrogen peroxide facilitates pulp brightness. The use of hydrogen peroxide in this industry has increased-to approximately 300,000 metric tons, which is 50 percent of current North American production capacity. Such usage will continue to increase significantly.
The use of hydrogen peroxide is expanding quickly in other markets as well, such as in water and waste treatment, mining, chemical processing, textiles and industrial cleaning. Current world wide production annually is 1.4 million metric tons, with a 7 percent annual growth rate.
Hydrogen peroxide production is controlled by a few chemical companies that produce it in large scale plants as a 70 percent concentrate. However, the highly oxidative characteristics of that level of concentration requires nearly immediate dilution to 50 percent concentration for safe transport. Ultimately, hydrogen peroxide is used in concentrations of approximately 5-10 percent. Typical H.sub.2 O.sub.2 production processes are based on anthraquinone reduction-oxidation chemistry. The typical process steps are (1) hydrogenation of anthraquinone working solution in a fixed bed reactor; (2) separation of the catalyst fines; (3) oxidation of the hydrogenated anthraquinone working solution by air in a multi-stage packed bed tower while simultaneously producing H.sub.2 O.sub.2 in the organic stream; (4) extraction of the H.sub.2 O.sub.2 from the anthraquinone working solution by water in a multistage counter-current extraction column process; (5) recovery and polish purification of the anthraquinone working solution, the accompanying solvents, and their recycle to the hydrogenator; and (6) recovery, polish purification and-stabilization of the H.sub.2 O.sub.2 product.
The typical process outlined above, disclosed in U.S. Pat. Nos. 2,158,525 and 2,215,883 to Pfliderer and Riedel, respectively is suitable for large scale production of H.sub.2 O.sub.2. However, the process is unsuitable for small scale production (500-1,000 metric tons per year) and medium scale production (5,000 metric tons per year). This is because the packed tower used for oxidation, and the column for H.sub.2 O.sub.2 extraction are very large and do not easily scale up or down for modularity and operational flexibility. For example, for a nominal 5,000 metric ton per year mini-plant, the oxidation-tower will have three beds 4.5 feet in diameter and 15 feet tall, stacked in series. The height of the equipment for this single oxidation process is more than 60 feet.
Conventional processes to extract H.sub.2 O.sub.2 from reaction liquor also has several drawbacks. Such processes utilize counter-current, multi-stage, liquid-liquid extraction of the anthraquinone working solution (AQS) with water. However, these procedures result in a very low (1:20, i.e., one part water extract to 20 parts of AQS) phase ratio between water extract and the AQS. H.sub.2 O.sub.2 concentration in the aqueous fraction has to be high in these processes so that subsequent H.sub.2 O.sub.2 isolation steps can proceed more economically. As such, typical extractors are multi-stage, very large in volume and difficult to scale down. These systems can be highly unstable and thus require a high degree of operational control. Finally, a certain amount of the polar solvents also enters the final aqueous phase in these processes. This results in contamination of the H.sub.2 O.sub.2 phase and ultimately, loss of the solvent. Typical extraction equipment has 23 countercurrent stages and is approximately 100 feet tall.
Efforts have been made to minimize ancillary reducing reactions (leading primarily to nuclear hydrogenation of the aromatic nuclei of the working solution) to prevent loss of solvent and anthraquinone feedstock (U.S. Pat. No. 3,009,782 to Porter). However, final fractions of H.sub.2 O.sub.2 still contain high levels of organic contaminants that require further isolation and polishing.
Finally, and not surprisingly, the costs associated with the typical highly capital- and energy-intensive, large scale hydrogen peroxide processes are passed on to low-volume end users. These end users would benefit from methods for producing hydrogen peroxide more economically.
A need exists in the art for a process to produce hydrogen peroxide without the concomitant capital costs and handling problems associated with current production schemes. The process would allow effective H.sub.2 O.sub.2 production in small plant environments and therefore would have a small size or footprint compared to the footprint of the "host" industrial site. Finally, the H.sub.2 O.sub.2 process would be as modular as possible with the ability for quick start-up, shut-down and turnaround, while also accommodating variability in production rates.